Teletypewriter frequency shift transmission



March 16, T954 J. H. MccoY v TELETYPEWRITER FREQUENCY SHIFT TRANSMISSION 2 sheets-shed 1 Filed April 1. 1949 m.. 5th.. .o

JOHN HARVEY MCCOY J. H. MCCOY March 16, 1954 TELETYPEWRITER FREQUENCY SHIFT TRANSMISSION 2 Sheets-Sheet 2 Filed April l, 1949 .mimi

JoHN HARVEY MGCOY A7TORNEY Patented Mar. 16, j1954 envases TELETYREWRITER FREQUENCY SHIFT 'EEANSMISSION massimi/femm John Harvey McCoy, Huntington, N. Yv., assignor to 'theUnited States of America` represented Aspiration Abri! 1 194s, serial Nq. 85,4112. series,- wlf. 1181-@ (Grantedunder 'riue 35, U. s. code (19.52.),

sec. 266) Thisis a continuation( in part of my, applicationnow abandoned Serial No. 509,999 led N- vember- 12, 1943, forthesarne subject matter.

The invention described and claimed herein may lbe manufactured and used by or for the Government for governmental purposes, Without th'epayment to me of any royalty thereon.

This' invention relates to improvements in devices for radio andl wire .telegraph teletypewriter communication and'the like.

"Appreciable improvement has been obtained in radio2"t'eletypewriter and telegraph communication by employing two distinct oscillators toprovideo'neradio frequency for the mark pulse and anotherfrequency "for the Space pulse. While this., method is' 'generallyeffectivefor radio teletypewriter operation, there is 'Stillan appreciable time 'of transition from one frequency` to the other, which is verynoticeableand, with thje ins'tantaneousrise and'fall'o'f the signals produces cli'clis and thumps.

Development of .frequencyshift?.` transmission was a further4 advance in which the output of a single transmitter is shifted from onefrequency toxanother to producer '.marksignals on one frequency, and space signals on thev other., but Wagfraught with similar and other objections, the mere instantaneous shifttendirl'g also topraduce .-clicl;s, and fthumpsf and there were difiiculties in maintaining stabilityof signal frequency. and amplitude levels. These. clicks and thumps are understood to be manifestations of wave harmonics produced'by asquare shift of frequency, analogous to effectsvmanifest in the transmission of a"square Wave Signal, and-understood from the principles involved in the application of the Fourier equations, discussed in various publications, as for instance, ,Glasgowfs Principles of Radiol Engineering,;-lvlcGraw Hill, 1936.

D ue principally to this invention, teletypewriter systems of. radio transmission (usingsignals alternated on two frequencies); 0.1 'which a eenerally used embodiment is rc0mmercially available under 'the trade name Teletype have been found a highly satisfactory., and eftective, means for radio communication over ,long distances, in- Giudice .transoceanic channels. in theace of, 'urifavorable climatic. and magneti@ `OilcitiOIlS,iii spaceV intervening ibetween transmitters ,and receivers (especially with diversity. retention), ac.- comnanied by advantases, irisecrecy and Qomnaratively. high speed-operations. Earlier. the teietynewriter systems were.v largelr. limited., t0

wire transmission. i.@armadio,cistimulantswerel operative and these were objectionable in so many Ways that none attained practical success.

It was found peculiarly difncult to overcome interference with communications in other channels produced by the mere frequency shift with asquare Wave equivalent of shift, and Wide use of teletypewriter apparatus was long limited to Wire lines on that account.` This diiiiculty is accentuated by the fact that in the ultimate development of teletypewriter mechanisms responsive to frequency shift signals, the receiving apparatus is dependent on each of the transmitted frequencies' to energize respective mechanisms, one for -space operations, and the other for mark operations. As the system is only valuable 'for its relatively high speed operations, the square shift .seemed atfirst essential, as it Wouldpermit an immediate establishment of the necessary energizing signal of ample duration on each frequency. Avoiding the click and thump by modifying the angularity of the shift, involves liability that sacrifice of `duration of the signals at at levels may occur to such extent that dependable operation of the two mechanisms is impaired. The lower harmonics are believed the principal offenders in producing the clicks and thumps, but whether so or not, my invention has been found eiective in reducing these effects to such extent that interferences are immaterial or obviated While retaining the requisite certainty of operation of the remote receiving mechanisms. As a result, radio teletypewriter communication utilizing the type of mechanisms indicated has multiplied greatly, fty and more transoceanic systems having been in operation at one time, in addition to many land radio teletypewriter channels.

TheV importance of the invention in its capability of meeting peculiar requirements of transmission and certaintyofI operation of electro-mechanical instruments at the receiving station may be further appreciated when the vagaries of fading and the inherentliabilities of phase disturbances found in prior conventional systems of raf dio signalling and reception are considered. The effects remedied in some degree by diversity reception are Well understood., and the value of the, stable levels of both frequencies transmitted by the instant invention in increasing the certainty of response of the receiving station mechanism Within the available code unit duraticn will become apparent from the disclosure.

While my invention may be applied to the operation of various receivers and electric type- .writers the. particular commercial, apparatus mentioned (Teletype) is representative of a class which has been found most satisfactory and is universally used. In these machines the sending station utilizes a keyer which transmits alphabetical, numerical, yand other characters, by means of a code, in which several signal shifts of the frequencies transmitted occur for each character, and the receiving apparatus responds to each code signal according to the number and order of the space and mark impulsesso received, to print a correspondingV character. It has been found advantageous to use the same number of units in each character signal, and the apparatus called the Teletype uses what is known as the five unit code; There have been developed seven unit code systems in other commercial systems but the essential functions and results are generally similar. i

According to current practice with the five unit code utilizing my invention, transmission of communications which are` fully written outthat is,'printed.-c.by,the receiver apparatus, has

`been carried out :atspeeds of to 100 wordsand moreper'minuta The vlimitation of speed is imposed by the capability of the. keyboard operator `Vand'k machines.: Manual operation of the keyboard'permits a rate of 40 to 50 words per minute.1. Speedsjupito 400 words per minute have been effected bynsing perforated tape records fedginto appropriate transmitter machines, and receiving the transmittedV radio communications ongwhat' are termedtape relay stations, producing punched tape, which is afterward fed into lower speed tape-operated electric typewriter machines, so that several such typewriters may 'be writing the material received over one channel and receiver at a tape relay station. Or the tape may be cut into parts and fed into transmitters operating on channels to respective widely separated destinations.

Considering the use of the common teletypewriter system at dwords per minute with the "iive unit code, seven impulses are required for cach character, because in addition to the iive units of the code, each character group begins with a space and ends with a mark Thus 450 characters and word spaces per minute would be involved, or 3,150 units of either mark or space per minute. The higher word rate speeds would involve correspondingly greater numbers of units These speeds compare with approximately 375 key movements per minute by fast manual key telegraphers.

From the foregoing an understanding may be derived of the great rapidity of the key shifts involved in practical operation of the invention, and the need for making the transmitted code units as long as possible and of exceedingly stable frequency at each limit of shift, in order that a dependable response of mechanical recording devices to each unit of each code symbol may be assured. A high degree of accuracy of transmitted information is manifest in the use of such systems embodying my invention, and it has been generally adopted by governmental and commercial radio teletypewriter communications systems. Using what is known as the Boehme keyer and receiver, rates from 400 to 600 words per minute have been attained.

It is an object of this invention to provide a system of radio telegraph and teletypewriter communication in which a definite steady signal of either one fixed frequency or an alternate one is always on the air, the carrier or assigned frequency being modulated by keying to produce an Y cords this change.

alternate one, so as to provide two distinct frequencies of iixed values for mark and space pulses respectively. The energy radiating does not leave the air when` a space? signal occurs, but shifts lower by 850 cycles (from a normal chosen in the range, say, between 1.8 megacycles and 6 megacycles, or higher), and the radio receivers beat frequency oscillator immediately re- By means of a system of detectors and lters at the receiving station the two signals are rectified, amplified and converted to resultant power in respective circuits which is made to actuate respective mechanisms or devices of recording machines, one form of which is called a printenl A further object is to provide a radio telegraph and teletypewriter communication system in which a transmitter is constantly excited by the output of a crystal oscillator whose frequency is modulated by a separately excited oscillator having frequency aected by reactance means controlled by keying to produce one frequency for a mark pulse and another frequency for a space pulse.

A further object is the provision of a radio teletypewriter and telegraph communication system in which a normally operating frequency is modulated by reactance-controlled means to 'produce a second predetermined frequency so as to provide one frequency. for mark pulses and another frequency for space pulses without introducing objectionable harmonics interference incident to the shift from one to the other.

These and other objects are attained by the novel method, apparatus and arrangement hereinafter described and illustrated by the accompanying drawings, in which- Figure l shows a block diagram of an arrangement embodying the invention;

Figure 2 is a representation of a typical series of signals for a single character capable of operating electrical typewriter mechanisms so as to print the corresponding character;

Figure 3 is a circuit diagram of an exciter circuit for a transmitter in which my invention is incorporated.

Referring to the block diagram of the drawings, the radio teletypewriter and telegraph system is shown to comprise a keying head or teletypewriter l, of any conventional type (for instance, the well-known Teletype sending machine) for operating an intermediate keyer 2, which may be a mechanical or an electronic keying device. The keyer 2 is connected to a reactance circuit unit or modulator 3 controlling the frequency of a self-excited oscillator 5, the latter being the sole direct-acting modulator of the output frequency, as will appear.

A shift control or limit device 4 interposed bctween the reactance unit 3 and the oscillator 5 or otherwise analogously associated with the reactance circuit, enables the frequency-shifting characteristics of the reactance modulator operation to be adjusted to different predetermined values, and particularly one establishing the chosen shift of frequency.

The output of a crystal oscillator 6 having a definite xecl frequency is mixed additively with the normal frequency of the self-excited oscillator to provide a signal of two limits of frequency, and one of which is always on the air. The output of the crystal oscillator is fed to a mixer 'i where it is mixed with the output of the self-excited oscillator. The output of the mixer is then passedl through amplifier 8 to excite the transmitter 9. On. this. ,accountv the elements numbered 3 tok 8 above, taken. together-may' be termed an exciter.

The crystal oscillator operates. at a frequency within a range comprising a. band of available channels with a` lower limit of about 1.8 megacycles, and. in each instance a few hundred. kilocycles below the desired lower output frequency of the mixer l; and the self-excited oscillator operates at a frequency equal to the exciter. output frequency minus the crystal, oscillator` fre-v quency.

The self-excited oscillator 5 in they specific instance chosen for discussion tends to operate constantly at 200 kilocycles and the normal exciting frequency for the transmitter is equal to the frequency of the crystal oscillator plus 200 kilocycles. After reaching the transmitter this exciting frequency may be multiplied.. The frequency of the crystal oscillator can be determined. by dividing the predetermined upper output frequency of the transmitter by the number of times the exciting frequency is doubled in the transi mitter and subtracting 200. That is, the crystal oscillator frequency plus 200 equals normal frequency 1 or 2 or 4 or 8 of the transmitter. The amount of frequency shift in the 200 kilocycles self-excited`oscillator In operation, the transmitter is normally con stantly excited by a frequency equal. to the frequency of the crystal oscillator plus 200 k-ilocycles,

the latter being 'the normal or mark frequency of the. self-excited oscillator. When aV key is closed or opened, as may be desired, the-reactance modulator 3 changes theA characteristics of the self-excited oscillator circuit and causes the frequency thereof to shift 850 cycles, or any other number of cycles as may be previously determined. As actually used, this shift is subtractive so that the frequency' of this oscillator becomes 199.15 kilocycles, for ak transmitter output without doubling. The extent' of shift offrequency can be adjusted byl the shiftV control 4. The shift in the frequency ofthe oscillator 5 accordingly causes a shift in the exciter output frequency, thus causing the transmitter to radiate the .alternate and lower one of'two different frequencies. It will be seen thatv there will be no material time of'transition from onefrequency to the other, as the transmitter is at all times radiatingA either one or the other ofthe frequencies. Accordingly, and additionally,qdue tothe interposition of the reactance tube between' the keyer 2 and the self-excitedv oscillator, there will be no clicks or thumps such as occur in the usual make and' break systemY or in the usual mere. shifting from one frequency to another by keying;

Considering more particularlyv the elements of the system, it may be explained that the sending keyboard indicated at I in Figure 1, isl inl practice a simple set of key levers resembling a typewriter* keyboard, by which, on depression of a given key,- a set of cams and selectors of proper function are operated to transmit at- 2 a nhmber (in the spe*- cic case illustrated, seven) of makeandjbreak These causer transmission' of; a number` of space and mar ."'impulses` in the required sequenceV toV represent the code pattern of,` andto cause prntin'gof; a particular' signals of special order.

character. (representedoxr thadepressedkeyboard.- key) at the remote printer,, or to. eiect. 3m01.1- trol, of. someY otherv machine. function. As before mentioned, a tapelrelay record. and correspond.- ing sending machinemay be. employed-in place of the keyboard, sothat a. more rapiditransmission of the code elements... is effected, and. either a printer or a` more rapidtape perforatorY employed. at the. receiving station; In either; case. the signal values on the air consistrof shifts. of.. the transmitted frequency from. low` tohigh; or vice' versa,v andl while the term: carrier has,Y been employed to designates the wave force. projected;y it may beunderstoodthat this termis usedf for convenience to indicate the assigned frequency for a transmitting station. It does not carry the usuala connotatiom, and; is? irr fact' av misnomer. since there is no. actual. carrier which ist menu--v lated While on the airA to:` giye significant signals, but there is simply either.A one fla-t frequency or: another,` andthe two are` independentr and; alter--` native:v

rIihe,y higher` of these frequenciesconstitutes.- af` mark signal, andthe lower one aspace'signal',

in: the general. practicepof the art'. Inr certairr character code groups. two; or.l more similar-r code units', Occur in immediate succession. (see: FigureV 2.) andl in. such case (using. az current formzofl tele. typewriter apparatus) the particular.Vr frequency'.- representing these units is simply maintainedz steadily fora period equal to the sumof the i'nteizvals of time which would berequired to trans mit. the samenumber. ofi. discrete units. Therecell/ing apparatus is nearly, butnctzexactly; syn,-v ohronized; in' certainessential; basic movements with.; corresponding movements'. of thee sendingv machine, so that setting or selector.;nlembersmov` ing in timed sequence inthe receiver will be successively brought to positions to respond to relatedzmark" or space signals ofthe same order, so; that; a.. group of members` may be selected or set up'. at the; receiving. machine corresponding to= the code group transmittedandy received; result'- ing. in; printing of a corresponding: character, forv instance. Illhe errory in synchronization is: pur-- pos'elyxmadein one direction, and a means-isA provided'. whereby the space unit: forming: the` beginning off each code group,- automatically syne. chronizes` the receiving machinen anewV with the sending machine.

While in the Teletype machine the initial send'- ingy circui'teclosing devices-especially those immediately responsive to keyboard=`operationare mechanical and to that extent correspondto make and break'keying, the frequencies of the makesand breaks greatly exceed thefrapidity of'custof mary make and break Morse key operations, as before pointed out; and at the relay 2 it is pos"- sible to use certain forms of electronic circuitinterruptingdevices, as, electron tubes, with .valve or othery functions applicable-to the changes of' the frequency'ofthe 200. kilocycle oscillator. This: Wouldbe especially advantageousin a; taper-relay'v stationk Where' the tape operated. machinesy proc-- duce characterv codeunits at much. higher-than` manual keyboard sending speeds.-

Since theL features of the' keyboard and the` mechanical codingmechanism: ofthe sending m`av chine mentioned-as' well as the" details of"A the code signal-responsiveprinter machineffor-receivingY stations-are well'known, -the details-ofthese are not illustrated. They' formncrnovel-"part ofi the invention, but thelrgeneral natureand-function are lcirefiy r referred# to* itr order' tliattheene-fr culiar .requirements of the present invention may be better understood.

Similarly, the circuit for the transmitter unit 9 of Figure 1 is not shown in detail, since circuits of this kind are well known and understood. These will vary according to the nature of the service involved, particularly as to distance over which the signal is to be transmitted, and the frequency, especially when the latter involves frequency-doubling of the output of the amplifier 8 of Figure 1. No features of invention are ininvolved in the transmitter when adapted to these requirements, and transmitters with doubling applicable to the instant use are available.

The exciter Referring to the circuit details of the invention shown. in Figures 1 and 3, it may be explained that in practice the units shown in these figures designated by numerals 2, 3, l, 5, 6, 1 and 8, are assembled in a single panel with an appropriate number of separate chassis each carrying the more closely coordinated circuit groups comprising the parameters required for basic functions of the invention. These chassis forms and groupings are not specifically defined in the disclosure since they are discretionary but the elements numbers 2 to 'i of Figure l are shown in Figure 3 as a unit, since this group is generally defined as an exciterf Giving attention to the details of the elements of the exciter which are more ,Specifically shown in Figure 3, specic details of power supply, voltage regulation, and teietypewrter transmission to the relay or electrode or mechanical keyer 2 of Figure i are thought unnecessary to illustrate and are omitted.

Crystal oscillator 6 rfhe crystal oscillator 6 is a Pierce type circuit,

and comprises a crystal selector switch unit H l in which three switch points are connected to one electrode of respective crystal units I2 of different frequencies chosen for predetermined alternative channel assignments. The arm I3 of the switch is connected direc y to the grid of a triode Vl and the latter is returned across a resistor Ri to ground, bypassed by capacitor CI. The plate of this tube is coupled by capacitor C2 to the outer electrode of the selected crystal. The cathode of this tube is returned to ground through resistor R2, bypassed by capacitor C4. The plate supply is, brought across an R.F. choke LI and resistor R11, and a R.F. bypass capacitor C5 couples the B supply to ground between this inductance and resistor R4, these cooperating with Li in avoiding disturbance of the plate current.

The mixer 7 In the mixer i--a balanced modulator-pentode tubes V2 and V3 are used in push-pull relation at their screen grids to respond to the 200 kilocycle oscillator frequency, and at the normal control grids in parallel response to the crystal oscillator output. They are coupled to the plate of the crystal oscillator tube Vi by capacitor C3 feeding to the rst injector or normal control grids of the tubes V2, V3 in parallel. This particular connection, in conjunction with other elements, has the effect of balancing out the crystal oscillations at the output of the mixer; but, in coaction with the input of the low frequency oscillator it develops the desired beat eiect, as is well understood. 'I'he last named two grids have a common leak resistor R3 returnedY to ground. The cathodes of tubes V2 and V3 are returned to ground through biasing resistor R5 with R.F. bypass condenser C5. The plates are connected directly to respective ends of the primary L2 of a transformer coupling the mixer output by secondary L3 to the amplifier. A center tap on this primary is connected to the plate voltage source by potentiometer arm i4 which wipes the middle one of three resistors R6, R8 and Rl connected in series between the suppressor grids of the two tubes, which serve as returns of the resonant take-off circuit. The arm i4 also provides an adjustment of relative voltages and plate circuit return, by which the eiective balancing out of the crystal frequency may be perfected. The last named two grids are also coupled by respective bypass capacitors C6 and CT to ground. In the potentiometer lead to the tap on the primary L2 last mentioned, the plate supply is connected thereto, across a resistor R9. Proper B voltages are thereby supplied to both the plates and suppressor grids of these two tubes. The primary L2 is bridged by variable tuning capacitor CS, the two constituting a parallel resonant circuit, which is tuned for the purpose of selecting one of the side bands produced by the beating together of the signals from the crystal oscillator ii and the 20G kilocycle oscillator 5.

The output of the 200 kilocycle oscillator is fed to the middle or second injector grids (normally the screen grids) 0f the two tubes V2, V3 of the 'mixen by coupling condensers CU! and C15, and those grids are returned to ground across respective resistors Rl2 and R13. The output from the mixer to the amplifier is taken from the secondary L3 of the coupling transformer, the two ends of which are incorporated in a twisted pair L3 to reduce inductive effects,

one end being returned to ground, and the other serving as the lead to the amplier.

The amplifier 8 The amplifier 8 may be of any form suitable for such purposes, of which a number of circuits are well known. The instant amplier receives the output of the mixer through the choke L1 and parallel resistor R2? i as a lter of undesired (parasitic.) frequencies) at the grid of an 807 tube Vi. The cathode of this tube is returned through biasing resistor Rl with R.F. bypass capacitor CH. Positive feedback is secured by a lead from the screen grid across a filter and series capacitor CHS to the cathode, the filter consisting of a choke L8 and resistor R28 in parallel. Resistor R32 and part of resistor RH are connected in series between the last named lter and the plate supply by potentiometer arm i5, the secon-d named resistor Ril extending beyond the arm l5 to ground, and the two resistors and lter serve as screen-dropping resistance. The plate supply arrives across the primary Ll of a coupling transformer, the secondary L5 of which serves as the coupling of the f output to the transmitter 9. A variable capacitor C12 is `connected between the plate and ground, as a tuning for resonance with the mixer output. The iast named capacitor and the last named choke are coupled in series parallel by a capacitor C13 to complete the tank parallelresonant circuit R.F. path. The tube VT is utilized additionally as a cathode follower in relation to a SES resonance indicator tube V8, the cathode of the tube V'i being connected across resistor`R24 to the 'grid of the resonance tube.'

The resistor RI I 'is connected in seriesbetween the plate Ysupply of tube lVI and ground, and is used as a potentiometer, its wiper I being in series with the resistor R32, filter L8R28 and screen grid of V7. This Wiper is also connected across resistor R to the cathode of the resonance indicator tube V8, as part of the stabilizing means.

The cathode of the later tube is also connected to ground with a biasing resistor RI1, in such relation that the eye of the beam is closed when the mixer and amplifier plate circuits are in resonant relation. This closure is dependent on proper adjustment of the side band tuning at the mixer and the output tuning at the amplier. The resonance indicator cathode is connected between the resistors RI'I and R9' and beyond this junction, across an interposed resistorvRSB, with the plate supply of the units 6. 1 and 8.

The 200 kilocycle oscillator V5 The 200 kilocycle oscillator 5 comprises the 6N? twin triode tube V4 in a push-pull self-excited oscillator circuit in which the tank com prises a variable choke coil L5 between the plates, having its inductance variable, preferably by using an adjustable slug (not shown), and a parallel variable capacitor CIB which is the normal frequency adjusting means, the slug being ordinarily undisturbed after initial setting. Positive feed-back from the plates is obtained across capacitors C22 and C23 to the opposite grids of the tube, and ybiasing grid leaks are provided at R22 and R23 to ground. The twin cathode lead of this tube is connected directly 'to ground Without intervening impedance. The 150 volt plate voltage for this tube is fed in parallel across symmetrical resistors R and R2I from a sup'- ply, the voltage of which should be stabilized. Capacitor C24 serves to bypass R.F. to ground and thus avoid disturbance of the plate supply. By adjustment of the parameters the output of this oscillator in the two leads Ato the mixer is symmetrical at the capacitors CI4 and CI5.

There thus become manifest in the mixer four major frequencies, as follows:

(l) The 200 kilocycle oscillator frequency;

(2) The crystal frequency; y

(3) A sideband frequency 200 kilocycles higher than the crystal frequency.; y

(4) A sideband Yfrequency 200 kilocycles lower than the crystal frequency.

oscillator frequency and the expected sidebands evolved, either of which may be then accentuated by tuning means C8, R8 and amplified. The higher band is the one selected, both for the high output or mark frequency and for the low or space frequency and this band is tuned by the capacitor C8 after an initial adjustment o'f the resistance R8. 1. Notwithstanding this tuning. .it is possible to effect a variation of the mixer eutput plus or minus 1 kilocycle, byva'riat'ion's at the oscillator 5 and a shift much less than this is effectedby the means to be described. The'shift is manifest by reactance applied to one lside ot the 200 kilocycle oscillator circuit. -But the modifying component is manifest in both sides of the oscillator circuit, so that a net change of the 'input to the mixer is attained having the desired frequency values at each limit.

The reactance circuit 3 The means for modifying the frequency of the 200 kilocycle oscillator is a reactance tube circuit of peculiar function specially designed and adapted to effect the desired end without coinplexity in the elements required, 'and eliminating movement in the system of essential members having appreciable mass or inertia, disregard'- ing these factors in the electron.

The frequency-changing means for the 200 kilocycle oscillator herein (see Figure 3) co'mprises a 6SJ7 tube V5 in the present instance although the circuit is adaptable to use of other tubes, as will vbe understood from this disclosure` This tube is operated from the same volt plate potential source as the 200 -kilocycle oscillator, with an interposed resistor RI 4 to prevent communication of material parasitic signal feed back to the line. The cathode is grounded directly so that a minimum impedance of response to signal input at the control grid is interposed there. The screen grid and suppressor grid are used conventionally. The control grid in this device has a tank circuit of novel composition adapting this tube and its connected circuits to function with peculiar benefit in the frequency shift system, particularly in preventing the clicks, thumps and other manifestations marking inter-'- ference with receivers tuned to other frequencies than those represented in the shift limits set up in the final output of this exciter.

The control grid is returned to ground through three resistors, RI5, RIE and RI-8 in series, the first two being of 100,000 ohms each, while the third, for instance, is of 10,000 ohms. Between the iirst two of these resistors (which may be called an intermediate high point the grid return resistance), a pulse-rounding condenser CI8 is coupled to ground in parallel with resistors RIB and RIO, and at a junction between resistors RI6 and RIB a dropping resistor RIS of 100,000 ohms is connected in a bias control lead from the plate supply voltage. In this relation, the -resistor RIB serves additionally as a bleeder `of current from the IR drop across RIS at the junction of the bias control lead and grid return. So much of the bleeder I8 as is effective atany given time regulates the bias of the tube V5, as will appear, -so that the tube conducts at a rate adjusted to produce a reduired freouency effectin the 200 kilocycle oscillator circuit, allowing the latter to oscillate at its full rated frequency, or such frequency xedly modified as necessary to produce the necessary mark high frequency at the output of the exciter. The V5 'plate is coupled without other impedance 'to one of the plate leads 0f the V!! tube by means of a 1,80 micro-microfarad capacitor CIS. In this aspect, the reactance circuit 3 is virtually a part of the plate tank circuit of the 200 kilocycle oscillator 5 as it forms a part of the reactance thereof, both by reason of the capacitor CIB and the small internal capacitance and by the impedance of the tube V5 and its circuits. Any change in the conductance of the tubevaries the reactance effective across capacitor CIG. In the use of lie device one such condition is a normal one and is xedly stable. Only one other condition in this respect is alternately manifest as in Figure 3, and that, too, is fixedly stable. Should the tube V conductance be increased, impedance of the tube is reduced, and lower frequency is involved in the oscillator. On the other hand, if conductance is reduced, as, by application of a negative-going potential on the grid of tube V5, impedance is increased, a higher potential is maintained at the near electrode, and frequency of oscillation in the platetank circuit of oscillator 5 correspondingly raised.

Means is provided for effecting the necessary changes of potential on the grid of tube V5 in response to key operations or other circuit closing and openingat the Teletype machines, the essentials of which are shown in Figure 3. This is a relay 2, which corresponds to the block element 2 of Figure l, as one conventional possibility.

This relay indicates the nature of the pulses by which the shifts of frequency of the 200 kilocycle oscillator are preferably effected. The relay is operated customarily from the teletypewriter automatic coding sending machine :by alternate opposite current pulse polarities in code circuits I1-iB, swinging the armature in respective directions. With the line closed at switch S1114 below relay 2, on movement of the relay arm to the right, the resistance-shorting shunt circuit i8 is closed to ground from an intermediate point on bleeder resistor RIB. This increases the bleeding of current from IR drop at RIS and lowers the potential eiiective at the grid of the reactance tube V5 in the circuit 3. This swings the grid less positive, reducing conductance in the tube, raising plate voltage and increasing effective reactance in the tank circuit (including CIG) of the oscillator 5 so as to raise its frequency. Opposite movement of the relay arm (to the left) at 2 opens the shorting circuit I9, making the whole of RIB effective, so that bleeding is reduced and a higher voltage made effective at the control grid of V5. This increases conductance in the tube and lowers its plate voltage, with a resultant lower frequency in the oscillator 5, and consequently in the signal output of the transmitter, in the manner before indicated.

The shunt connection from the relay 2 is made at a wiper arm operating` on the resistor Ri which thus becomes a potentiometer the arm shorting out the ground end portion of this resistor when the relay is closed, and being adjustable to determine the rise in frequencyY caused on closing of the relay 2. This changes reactance time at Cl and produces the lower or space frequency signal as before indicated. The arm.

at RIS remains in a permanent position after all parameters are adjusted, so that a definite rise in frequency is produced in the oscillator 5 on each closing of the relay, and a like permanence of low frequency is involved by the values of the resistors when the shunt circuit is opened at the relay. Some adjustment of the level of frequency might be obtained by making the series resistance R! 5, Ri, RIS variable independently of the shunt through the arm, but it is found more practicable to leave these fixed and use the adjustments at RB and C8 of the mixer, and at L6 and CIS of the 200 kilocycle oscillator.

In the use of this apparatus, if the assigned frequency should be four megacycles, for instance, this may be satisfied where a single doubling at the transmitter is employed by selecting a crystal at the switch I I which gives the crystal oscillator circuit a frequency of 1,800,212-5 cycles and the self-excited oscillator at 5 would normally oscillate at its 200 kilocycle frequency, this beating against the crystal frequency in the mixer so that the additive side band filtered out will have the "mark value of 2,000,2l2.5 cycles as it reaches and leaves the amplier. In case the crystal is not exactly of the value desired, this may be compensated for by an adjustment of the unit 4 or the side band tuning at C8, Figure 3, or both, or use of the tuning in the 200 kilocycle oscillator with or without adjustment of the resistor R8, in the mixer 1. This frequency after amplification is then doubled conventionally in the transmitter, producing a mark signal frequency of 4,006,425 cycles. The shiftcontrol at potentiometer l is then adjusted so that with the reactance in circuit 3, on closing operation at the relay, the frequency of the self-excited 200 kilocycle oscillator is shifted by the introduced impedance at the grid in circuit 3 downward to 199,575 cycle s by the introduced reactance at circuit 3. This added to the crystal frequency by the mixer produces the intermediate frequency of 1,999,787.5, which is ampliiiedy and doubled once in the transmitter to produce the space signal of 3,999,575 cycles on the air. By this function of the system it will be observed that the assigned frequency is never on the air, but the two alternate frequencies are transmitted in such manner that no intermission occurs exceeding the interval in either signal frequency, or sufficient to cause impairment of efficiency of transmission and reception, or to cause interference at receivers tuned to different channels or frequencies.

Should compensation be required for variation of the crystal from the desired frequency therefor, the self excited oscillator, may be correspondingly adjusted :by the arm of the resistor RIB at Ll and the sideband filter or resonating adjustment at the mixer I so that the desired outputs to the amplifier from the mixer may be secured having the required frequencies. The adjustment thus available permits a tolerance of 1 kilocycle plus or minus in the value of the selected crystal. Of course, in the case of an arbitrary assigned frequency some variation from the exact upper and lower limit frequencies is thus possible within the permissible frequency error limits set by governmental -regulations so that the upper frequency transmitted may be slightly more than 425 cycles above that assigned, or the lower transmitted frequency may vary correspondingly. However, in order to avoid difficulties in reception an exact adjustment to the ideals on the air with a difference of 850 cycles is desirable, and is the conventional practice.

It should be appreciated that the selection of the values of the capacitors CIS and CiS is a material factor in enabling the optimum of effectiveness and rapidity of shift from one frequency to the other at the same time that the tendency toward a square Wave form in the shift with attendant prohibitive interference manifestations is modified. The condenser C I 8 also has a phasing function contributing to this result,

lattire extreme corner that the objectionable a'etatoc lf3 manifestations of simple make and 'break' forms would be vapproximated. With the value indicated'in this capacitor, howeven'it has been found that a rounding at the extremes of the shifts is produced sufficient to reduce interference to innocuousness, while yet retaining a practically vertical rise in the major portion of the shift, with a flat-top or bottom of relatively long duration, Vac` cording to the direction of the shift.

It will be seen in the disclosure made, that, while the shift is initially dependent yon a make and break of a circuit at the relay, the actual change in the self-excited oscillator circuit is not due toa simple abrupt change from an open circuit tovclosed circuit, or vice versa, `but to an electronic change in conductance in an element of a collateral circuit, modified in a novel relation to certain interference factors so that the ylatter are suppressed, which they would not be if the make and break were directly in one of the circuits of the oscillator corresponding to the lprior practices mentioned in the introduction to this specifica-tion.

In discussing the shift from one frequency to another in the operation lof the system disclosed, the shift from the lower frequency to the upper one may -be said to be a pulse having a rise time, and the shift from the upper frequency to the lower one may be said to have a fall time. In a major intermediate part of each of these rise and fall times, the time is inappreciable and the completion of the major part of the shift is practically instantaneous. At the start of each shift, however, owing to the explained function of the capacitor C18 and resultant effect on the actance at C16 accompanying the change, and

the rise time and fall time may be said to begin with key action at relay 2 and to continue to the completion of the shift by action of the tube and its tank. This time will be small and the curve representing the beginning of shift and these times would closely approximate one tangent to the top or bottom of the shift and tangent to a vertical rise or fall represented at the side of the pulse form, the curved part having a time value in the neighborhood of one-tenth more or less the rise or fall dimension of the pulse form. But in any event this curve is only manifest materially at the extreme beginning and end of the side of the pulse produced on each shift.

The form of this curve can be mathematically computed using principles of the Fourier analysis.

As before indicated the resistor RI 8 or so much of it as is effective on closed key at relay 2, serves as a bleeder by which a potential of moderate voltage is derived at the drop across RIB, and cooperating with resistances RIE and Rl 6 in the biasing of tube V5.

In operation during teletypewriter transmission, the unit 3 functions substantially as follows:

At open-key condition at relay 2, all of Rl as well as R15-RIS is effective. Less bleeding to ground will occur at.the RIS drop, and the higher voltage level will be manifest at the end of RIS, which will be accompanied by a higher grid potential and maximum conductance in the tube. At initial opening of the relay at 2, when the grid swings more positive, the function 'golf capacitor CIB becomes effective by charging performing a phase modifying function yat tlie grid, 'so `that the vrise of conductance 'is yrelatively gradual at lthe start, and involves roundingof the `corner of the pulse form of "the shift signal, since the capacitor is thus effective over only a4 small part of the pulse time or the code unit duration time. On closing of the relay 12, part of RI8 is shorted to ground increasing the bleeding of current from the potential at the drop across RIS, lowering 'voltage there, with corresponding effect at the grid of the tube V5 and consequent reduced conductance in this tube. But the reduction of vgrid potential and fully lowered conductance is delayed by discharge from capacitor C18, 'so 'that the beginning of the signal `shift to high frequency starts gradually, increasing exponentially over a short part of the code unit time producing an effect similar to the form of the corners of the shifts represented in Figure 2. The reduced potential becomes quickly stabilized at the Rlt drop and the top of the shift becomes substantially flat.

The system is adapted for use in facsimile or picture `transmission by radio -as well as radio telegraph and teletypewriter systems, although in their uses the relay 2 'specifically disclosed would be replaced by vapparatus appropriate to theuse and not comprised in the invention hereinafter claimed.

While the invention has found its `greatest value in enabling the practical use of frequency-shift transmission in radio teletypewriter systems, it may also nd value in wire transmission teletypewriter operation if several frequency ranges or channels are used on the same wire circuit. A single channel wire system may operate without causing difficulty by interference with radio receivers when a simple make Yand break shift between different frequency transmitters is practiced -as in the rst days of the use of the cornA marcial system known as the Teletype However, if several such sets of transmitters operating at different levels of frequency are used on a single wire line, and respective receivers are Vconnected to the line tuned to respond to these respective sets of frequencies, the clicks and thumps representing harmonics produced by square Wave eects, are liable to impair operation of all the systems or channels involved on a given wire. My invention then becomes important in enabling multichannel teletypewriter and like frequency shift communication on the single Wire circuit.

The above description is to be considered as illustrative and not limitative of the invention, of which modifications obviously may be made. While specic frequencies have been set forth, clearly other frequencies and other oscillator control within the scope of the claim may be eme ployed without departing from the spirit of the invention.

I claim:

l. In a frequency shift for oscillators of the character described wherein the oscillator includes a resonant circuit including capacitance, and having an output circuitacross said capacitance effective to control the frequency of the oscillator, a reactance tube circuit comprising essentially a triode having a plate circuit, a capacitor coupling the plate circuit to the resonant circuit of the oscillator, a maximum external resistance in series with the grid and cathode of the reactance tube, an electrical positive biasing source connected intermediately of said resistance in series with the remainder of said re sistance, grid and cathode, a condenser connected frequency of the latter, the output of the oscillator applied to a mixer with the output of a high frequency oscillator and a filtered sideband of the mixer output transmitted as radiated energy; said keying unit comprising a vacuum tube having an anode, a cathode and a control grid; an operating potential voltage source in series between the plate and cathode, a capacitor having one electrode connected directly to the plate and the other adapted to be connected in coupling relation to the said resonant oscillator circuit, a resistance in series between the plate and cathode, a grid return resistance between said grid and the negative of said operating potential source, a pulse rounding capacitor. coupled between an intermediate high point of said grid return resistance and the negative of said operating potential source, a snorting connection between a low value intermediate point in said grid return resistance and a more negative point in the grid return, a small positive potential applied to the grid return resistance intermediately of the part thereof between said snorting connection and the connection with said pulse-rounding capacitor, and a make and break keying device in the said snorting connection.

3. In a radio communications system wherein pulse code signals of different frequencies and predetermined unit duration are transmitted in immediate succession by frequency shift of oscillations in a normally stable oscillator circuit of a transmitter exciter; a keying system to shift the frequency of said oscillator comprising an electron valve device having an anode, a cathode, and a control electrode, a principal amplitude voltage source in series externally between the cathode and anode, an external grid return including a resistance, a bias control voltage source connected to the grid return resistance at a junction intermediately of the latter in series with the terminal part of said resistance, said terminal part constituting a bleeder resistance, a resistor in the line between the last named source and said junction, a pulse-rounding capacitor connected to the grid return resistance intermediately of the part between said junction and the grid, said capacitor in parallel with the terminal part of the grid return beyond said connection with the capacitor and in series with said bias control voltage source and intervening portion of the grid return resistance, a resistance-shorting line connected intermediately to the bleeder resistance parallel to a predetermined part of the latter to short out the last named part at times, and a circuit closing and opening device in said resistance shorting line operable at will, said capacitor, bias control voltage source and resistance components efective therewith having a time constant Value effective to produce a rounding-of the start of each change of bias incident to operations of said circuit closing and opening device, and such that said rounding extends over only a small part of the said unit duration of said pulse code signals.

JOHN HARVEY McCOY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,074,440 Usselman Mar. 23, 1937 2,103,847 Hansell Dec. 28, 1937 2,212,548 Noxon Aug. 27, 1940 2,225,691 Finch Dec. 24, 1940 2,349,811 Crosby May 30, 1944 2,403,358 Gerhard et al July 2, 1946 2,476,141 Goddard July 12, 1949 2,492,791 Finch Dec. 27, 1949 2,492,795 Goldstine Dec. 27, 1949 2,531,103 Beckwith Nov. 21, 1950 

